In applicant's co-pending application Ser. No. 275,257, now U.S. Pat. No. 3,801,307, the disclosure of which is hereby incorporated by reference, there is disclosed a process for reduction of reducible metal compounds wherein the reducible metal compounds and stoichiometric amount of reducing metal are introduced into a sealed reaction zone and heated to a temperature which is above the melting point of the reducing metal but below the temperature at which a reduction reaction between the reducible metal compound and the molten reducing metal will spontaneously occur. In this temperature range a reduction reaction is initiated between the reducible metal compound and a molten reducing metal by suddenly disrupting the surface of the molten reducing metal and allowing the reduction reaction to continue to completion. This process is a highly advantageous and economical process. However, it has been found especially where relatively large reaction masses are employed the product metal becomes molten from the heat of the reaction and adheres to or alloys with walls of the reaction vessel, which is usually ferrous in nature, thereby sometimes causing undesirable contamination of the product metal. For example, titanium metal attacks iron at a temperature of about 975.degree. C. Since iron is an undesirable contaminant for most purposes, the resultant iron-titanium alloyed is undesirable.
In addition, even when the reactor wall is a metal which does not form an undesirable alloy, for example, the same metal as is being formed, the intense heat of reaction tends to weaken or burn through the reactor wall, where the wall is not protected. U.S. Pat. No. 2,890,111, for example, describes cooling the reaction wall. However, this procedure retards the formation of a coherent fused plaque or button of metal.
The concept of a porous inert insulation layer between the reacting components and the side walls of a reactor has been described in the art, for example, in the preparation of vanadium and other metals by the thermite process (Z. anorg. Chem. 64, 217-24, CA 4, 874, 1910). Insulating layers have also been used for the reduction of uranium and thorium and are described by H. A. Wilhelm et al, Journal of Chemical Education, Vol. 37, page 56, February 1960.